In the construction of micromachined structures on a substrate, there are situations in which the structure has one or more members, such as beams, that are meant to spaced away from the substrate. For example, there are accelerometers that employ micromachined polysilicon structures as sensors to detect and measure acceleration. These sensors operate on the basis of changes in the differential capacitance of the sensor caused by the change in the relationship of sensor elements, such as the changes in the relationship of a movable beam with respect to two fixed beams (or plates).
To maximize the capacitance, the movable beam may contain numerous fingers that are interleaved between fingers of the two fixed beams. In the initial state, each finger of the movable beam is positioned midway between the fingers of two fixed beams so that one capacitor is formed by the finger of one fixed beam and the finger of the movable beam and a second (and equal) capacitor is formed by the other fixed beam and the same finger of the movable beam. Various shapes and arrangements of these elements have been used for the accelerometer sensor.
The application of a force along a sensitive axis of the accelerometer causes the fingers of the movable beam to move relative to the fingers of the fixed beams, causing a change in the capacitance, and a signal appears on the movable beam that reflects this amount of acceleration. Thus, for proper operation of the accelerometer, the movable beam must be free to move in response to accelerations experienced by the accelerometer.
"Stiction" or adhesion with respect to microstructures occurs when an element, such as a movable beam, becomes stuck to the substrate. Once a beam is stuck, it renders the sensor virtually useless. The cause of stiction can range from static to surface charge. It has been a challenge for microstructure designers and fabricators to overcome the stiction problem.
Referring to FIG. 1, a top view of a prior art accelerometer is shown generally at 100. The accelerometer has substrate 102 to which a micromachined sensor structure is attached. The sensor structure has fixed beams 104 and 106, and movable beam 108. Fixed beam 104 includes fixed fingers 110, 112, 122, and 124. Fixed beam 106 includes fixed fingers 114, 116, 118 and 120. Movable beam 108 includes center member 138 that has one end that connects to the middle of perpendicular disposed, elongated end member 139 and the other end that connects to the middle of perpendicular disposed, elongated end member 140. End member 139 is anchored to substrate 102 at 130 and 134, and end member 140 is anchored to substrate 102 at 132 and 136.
Center member 138 has movable fingers 142, 144, and 146 disposed perpendicularly from one side and movable fingers 148, 150, and 152 disposed perpendicularly from the other. The movable fingers are either between the fixed fingers of fixed beams 104 and 106, or adjacent a fixed finger of fixed beam 104 or 106.
The construction of the movable beam allows it to move in directions "A" and "B" under inertial loading. Since the stiffness of end members 139 and 140 may be varied, the amount of acceleration that it takes to deflect the center member 138 a measurable amount can be varied to meet various acceleration loading situations.
Referring to FIGS. 2 and 3, the fixed and movable fingers have a plurality of vertical spacers. The vertical spacers are used to try to overcome the stiction problem. In FIG. 2, movable finger 146 has vertical spacers 184, 186, and 188, and movable finger 144 has vertical spacers 202, 204, and 206. Fixed finger 110 that connects to substrate 102 at 180 has vertical spacers 190, 192, and 194, and fixed finger 120 that connects to substrate 102 at 182 has vertical spacers 196, 198, and 200. As shown in FIG. 3, the vertical spacers extend below the normal bottom of the fixed or movable fingers to perform the spacing function.
The mask that is used for producing the vertical spacers has 4.0 .mu.m wide openings traversing each row of fixed and movable fingers. After etching and a subsequent deposition of material, a finger, such as fixed finger 110, has vertical spacers 190, 192, and 194. The footprint of each vertical spacer is a flat surface that is approximately 4.0 .mu.m.times.4.0 .mu.m.
The vertical spacers shown in FIGS. 2 and 3, however, do not solve the stiction problem because, at times, the large size of the footprint allows the beam to stick to the substrate. Presently, there is not a method of making vertical spacers small enough to prevent the stiction problem.
The present invention overcomes this and other problems as will be set forth in the remainder of the specification referring to the attached drawings.